Software defined radio (SDR) combines hardware and software technologies that enable reconfigurable system architectures for wireless networks and user terminals. SDR provides many improvements over traditional hardware radio and software-controlled radio. In a hardware radio, the radio is implemented using hardware components only and cannot be modified except through physical intervention. In software-controlled radio, (SCR) only the control functions of the radio are implemented in software, and thus only limited functions are changeable using software. In contrast, SDR technology uses software and digital processing to perform the functions performed by analog hardware components in hardware radio and SCR. The degree to which these functions are moved from hardware to software varies from implementation to implementation.
As SDR functions migrate from fixed-function analog hardware to digital implementations, concepts which work well in the analog domain pose unique problems in the digital domain. For example, the super heterodyne or dual-conversion transceiver receives a signal that is downconverted to baseband in two stages. This two-stage receiver and transmitter architecture uses an RF block to convert an incoming signal to an intermediate frequency (IF) where image suppression and channel selection are performed with a narrow channel-select filter, such as a SAW, or ceramic filters. The now-filtered signal is then further downconverted to the baseband frequency. This radio architecture provides excellent sensitivity and selectivity characteristics. This comes at the expense of more complexity and cost, for such radio implementation typically requires an RF chip and an IF chip as well as discrete SAW filters and VCO/synthesizers.
An alternative to the super heterodyne receiver architecture is the Zero-IF receiver architecture, also known as a single-conversion, directs conversion or homodyne receiver architecture. Zero-IF receiver architecture enables direct conversion of RF signals to baseband without the use of an IF. Zero-IF architectures reduce component count, cost, system complexity, size, and power consumption. Despite these benefits, amplitude modulation (AM) current implementations of Zero-IF architecture fail to work well.
An envelope detector operating at either the carrier frequency in the simplest AM radios or, more typically, at an IF in super heterodyne radios are included in AM detection implementations. However, envelope detection in Zero-IF architecture has not been efficiently implemented because the Zero-IF receiver demodulates the incoming signal from RF to DC. Envelope detectors do not perform well at low frequencies near DC. This is particularly true for digital signals using AM modulation schemes, as the wide bandwidth renders cascaded integrator-comb (CIC) decimation filters less effective and also results in an image frequency problem. Ripple and negative peak clipping also are common issues associated with envelope detection at low frequencies near DC.
An alternate solution for AM detection employs a synchronous detector. However, the synchronous detector also has shortcomings. If there is any discrepancy between the actual RF carrier frequency and the receiver's local oscillator frequency, as occurs when the system is not tuned accurately, the output signal will not be centered. Temperature and manufacturing process variations can also introduce discrepancies between desired and actual frequency output. Regardless of cause, any frequency mismatch can introduce several unwelcome effects. For example, incorrect phase can cancel the output signal completely, resulting in no signal, or the output could be totally garbled because the reflected LSB harmonics are not aligned with the USB harmonics. Additionally, an overpowering low-frequency tone can be generated by the offset carrier.